qubes/tools: better handle qvm-template-postprocess called as root
qubes/tools: better handle qvm-template-postprocess called as root

This tool by design is called as root, so try to:
 - switch to normal user if possible
 - fix file permissions afterwards - if not

QubesOS/qubes-issues#2412

woju/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin